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Inconel 718 samples were printed with varying scan speed and laser power with volume energy density (VED) in the range of 52 J/mm3 to 75 J/mm3. Samples were then subjected to solution treatment and aging as per standards. Further, samples were characterized for their microstructure and metallurgical defects using optical microscopy (OM) and scanning electron microscopy. OM revealed that columnar grains had grown epitaxially in the build direction and grain size varies from bottom to top. Microscopic analysis on the XY plane (laser focusing plane) and the XZ plane (the plane in build direction) had revealed the presence of defects like metallurgical pores, interlayer cracks, open pores, lack of fusion, balling defects and their different morphologies. The process parameters such as scan speed, laser power and VED have influenced the porosity % and pore density and their distribution in the printed material. The microstructures also revealed the presence of oxides of Cr, Nb, Ti, Al and Fe which were identified near the interlayer cracks, large open pores and clusters of pores. Microhardness measurements of the printed samples were higher than the wrought Inconel 718 by 15-18%, both in the solution treated and aged conditions. The microhardness survey along the XY and XZ planes yielded different values even after double aging heat treatment. Average microhardness on the XZ plane was 15-20% more than that on the XY plane in each sample pertaining to the difference in grain morphology on the two planes. This difference in the microhardness values was less for the sample of high VED. The results suggest that VED in the range of 65 J/mm3 to 70 J/mm3 with scan speed and laser power between 860 mm/s to 970 mm/s and 260 W to 300 W, respectively, result in samples with tolerable microscopic defects.

In the present study, detailed metallurgical characterization was carried out on 3D-printed samples in as-printed and heat-treated conditions to understand the effect of scan speed and laser power on the microstructures, phases, types of defects, distribution of defects and oxides associated with defects. Extensive microstructural analysis was carried out using various analytical techniques such as optical microscopy, scanning and transmission electron microscopy and x-ray diffraction to establish a correlation between the microstructure phases and defects in the as-printed and heat-treated samples. A grain size study on heat-treated samples revealed that the grain size of columnar grains has increased with an increase in volume energy density (VED). The generation of different types of defects was correlated to the VED used to print samples. The optimum process parameters based on the analysis of defects through optical microscopy are: v = 860–960 mm/s; P = 250–300 W; VED = 65–70 J/mm3. Based on detailed microscopic analysis, it was noted that the heat treatment as per AMS5664 was inadequate and needs to be optimized for 3D printed IN718 samples due to incomplete dissolution of segregated Nb and Mo, carbides, oxides into the matrix during solution treatment.

The present study is an attempt to model dynamic recrystallization (DRX) in a single phase metal using a mean field crystal plasticity (MFCP) based approach. A new empirical equation is proposed to model nucleation, in which the nucleation rate is a function of microstructure and plasticity descriptors that are known to affect DRX behavior, such as the temperature, strain rate, grain fineness and stored energy. Grains undergo nucleation when their dislocation density exceeds a threshold value. Subsequently, new grains grow based on the difference in stored deformation energy with respect to the average value over all grains. The MFCP-DRX model is able to correctly predict trends for the flow stress, dislocation density evolution, grain size evolution and kinetics across a range of temperatures and strain rates for uniaxial compression. Transition of the flow stress from single to multiple peaks is observed with increasing temperature and decreasing strain rate, thus comparing well against known DRX trends. The evolutions of crystallographic texture during DRX in uniaxial compression and plane strain compression are compared against experimental observations. A sensitivity analysis is conducted to understand the effect of variables on nucleation and growth. The competition between nucleation and growth dominated deformation in different strain regimes is analyzed in detail across various temperatures and strain rates.

M300 maraging steel was fabricated by selective laser melting (SLM), and the effect of heat treatments on the corrosion and stress corrosion resistance of the alloy was evaluated in 3.5% NaCl solution. The microstructure of the alloy in the solution-treated and aged condition is typical of lath martensitic structure along with metallurgical defects typical of SLM-processed material. The results indicated that M300 steel processed by SLM is highly susceptible to stress corrosion cracking (SCC). The fracture morphology of the samples exhibited brittle intergranular cracking along with ductile cracking features in NaCl. HIP treatment after SLM and before solution treatment and aging improved the SCC resistance of the alloy in terms of marginal increase in elongation in NaCl solution. However, this treatment could not completely eliminate the SCC susceptibility of the maraging steel in NaCl solution. The SCC susceptibility of M300 maraging steel processed by SLM can be attributed to the presence of defects such as pores which assisted the early initiation of cracks and the resultant intergranular (IGC) failure. The defects were considerably healed by subjecting the SLM-processed alloy to HIP treatment, thus improving the SCC resistance. In addition to the reduction in pore density, the increased austenite content present in the alloy after heat treatment played significant role toward improving the corrosion resistance of the as-printed M300 steel.

The purpose of the present work is to study the mechanical behaviour of AA2219-T87 friction stir weld (FSW) joints at different temperatures (Room temperature (RT), 77 K and 20 K) and correlate them to the microstructures evolved during the joining process. For this purpose, standard full length tensile specimens as well as miniature tensile specimens extracted from different microstructural regions of the joint such as weld nugget zone (WNZ), thermo-mechanically affected zone (TMAZ) and heat-affected zone were used. Similarly, fracture toughness (FT) was evaluated by placing notches at the three microstructurally different regions. The tensile properties at 77 K and 20 K were higher compared to those at room temperature. Miniature tensile specimens extracted from WNZ and TMAZ regions showed similar strength properties; however lower hardness and ductility were observed in TMAZ. Failure of standard full length samples was always in TMAZ since it has lower hardness and ductility, as a result of strain hardening. The higher FT noticed in WNZ is attributed to the presence of very fine grains. The lower FT observed in the TMAZ is attributed to combined effect of the absence of strengthening precipitates and recrystallization. Comparison of the mechanical properties of AA2219-T87 FSW and TIG weld joints revealed superior tensile properties at room and cryogenic temperatures for FSW weld joints, thus making FSW, a better alternative for the fabrication of large sized earth storable and cryogenic propellant tanks of satellite launch vehicles.

18Ni-2000 MPa maraging steel (M300 grade) is extensively used for performance critical aerospace applications where strength-toughness balance is the basis for material selection. The key to achieve the desired mechanical properties in the final product is to process the material under conditions that will impart an equiaxed, fine grained microstructure. This in turn is controlled by the selected deformation processing conditions such as strain, strain rate and temperature. Therefore, hot deformation behavior of the material is usually studied under wide temperature, and strain rate ranges to map the microstructural evolution with specimens deformed to a given strain. However, the effect of strain on the microstructural evolution, its effect on the initiation and termination of dynamic recrystallization are essential to obtain balanced mechanical properties. In this study, analysis of stress-strain curves was conducted with an intention to obtain fine prior austenite grain (PAG) size through dynamic recrystallization (DRX), and the same was verified experimentally from the microstructures evolved through hot isothermal compression tests on cylindrical specimens subjected to different strains. A single peak type DRX curve was identified for analysis from the high temperature stress-strain curves. The theoretically determined critical strain was used to experimentally verify the initiation of DRX (DRXI) and transition from DRX dominant region to grain growth dominant region (DRXT). Hot isothermal compression tests have been conducted at T=1100°C and e = 0.1s−1 and obtained PAG size of 7.68 µm in the specimen deformed to theoretically determined optimum strain of 0.6, thereby validating the used models.

XH 67 is a nickel-based superalloy, which is used in critical parts of modern aerospace engines. In the present study, XH 67 specimens in five different heat treatment conditions selected based on the processing and application have been investigated for their microstructures, morphology, and composition of the matrix and second-phase particles. Optical microscopic (OM) analysis revealed the presence of bi-model neck-lace type grains and equiaxed grains as a result of different heat treatment conditions. Scanning electron microscopy (SEM) and elemental composition analysis revealed precipitates and second-phase particles. Carbides such as Ti (N, C) and (W, Mo, Ti) C were observed having different sizes and shapes. Cr-rich carbide phases were found to be preferentially segregated along the grain boundaries. X-ray diffraction analysis was carried out for assessing the phases present in the alloy. X-ray diffraction analysis revealed the gamma matrix and the precipitate phase are crystallographically identical (coherent). Transmission electron microscopic (TEM) analysis of the specimens confirmed the presence of uniformly distributed spherical Ni3 (Al, Ti) precipitates which impart thermal stability to the superalloy. Presence of different types of carbides is also revealed by TEM analysis.

The development of microstructure in the course of warm deformation of niobium-10 hafnium-1 titanium (wt%) alloy (C103) was studied by plane strain compression (PSC) testing in the range of 500–650 °C and two strain rates (ἑ) 0.01 and 1 s−1 which are commercially being practiced for rolling of the alloys. A total of 75% reduction in thickness was imparted to the samples during the warm deformation. The purpose of the study was to establish the warm rolling process parameters (ἑ and T) useful in the optimization of thermomechanical processing schedules to realize thin sheets of this difficult to process refractory material. Weak softening was noticed at temperatures of 600 °C (~ 0.35Tm) and above in the stress–strain plots. Partial dynamic recrystallisation was observed at the regions where maximum strain was observed in the sample during deformation at high temperatures as evidenced by electron backscatter diffraction (EBSD). Samples deformed in the temperature range of 500 °C to 600 °C and at ἑ of 1 s−1 showed dynamic recovery. High angle grain boundary fraction is higher for the sample deformed at a temperature of 650 °C and ἑ of 0.01 s−1 compared to the sample deformed at a ἑ of 1 s−1. Based on detailed microstructural observations, it was concluded that C103 alloy can be warm rolled at a temperature of 650 °C and ἑ of 0.01 s−1 to obtain localized dynamically recrystallized grain structure in the material and it is expected that a dynamic recrytallization will further advance with increase in reduction during deformation.

Superalloys are used in the strategic sectors of aerospace, defence and nuclear in view of their high specific strength at elevated temperatures, good weldability and excellent oxidation resistance. Inconel 600 is a nickel–chromium alloy with good oxidation resistance at higher temperatures and resistance in carburizing and chloride containing environments. Tungsten inert gas (TIG) welding is the most versatile fabrication process used due to its efficiency and high level of process control. However, the depth of penetration obtained in the weld joint is quite low making multi-pass welding inevitable for thick-section welding. To improve depth of penetration of TIG welded joints, flux bounded tungsten inert gas (FBTIG) welding was developed, that uses thin layer of activating flux on top of the surface of the workpiece, by maintaining gap along the weld line. The present work focuses on the effect of nature of flux and flux gap on the weld bead geometry and mechanical properties of FBTIG welded Inconel 600 alloy. Optimization of weld and flux parameters was carried out through bead-on-plate welding and butt weld joints of Inconel 600 was fabricated using the optimized parameters. Radiographically qualified aerospace quality welds were fabricated and depth of penetration has increased three times for FBTIG welds coated with SiO2 flux using flux gap of 2.5 mm. The room-temperature tensile strength of FBTIG weldments is comparable to that of TIG welds and it was concluded that single-pass FBTIG welding can replace multi-pass TIG welding, to join thicker sections of Inconel 600.

Microstructure evolution during hot working of Nb-10Hf-1Ti (C-103) alloy has been studied at various temperatures (1273–1473 K) and strain rates (0.01–10 s−1) using a Gleeble™ thermo-mechanical simulator. Flow behavior of the material has been analyzed, processing maps were generated and correlated with the microstructures. Flow stress variation with increasing temperature and decreasing strain rate has been very minimal except in the higher temperature and lower strain rate regime. Enrichment of Hf, C and N was observed at the second phase particle- grain boundary interface in samples deformed at lower strain rate and high temperature. Presence of Hf(C,N) was confirmed through SEM–EDS and these second phase particles may result in high resistance to deformation at this regime depending on their size and distribution. Lower temperature working in combination with static recrystallization or higher temperature working with inherent dynamic recrystallization can be employed for deformation. Strain rate of 0.1–0.01 s−1 and temperature range of 1373–1473 K is identified as a safe working zone for this alloy under a protective atmosphere.

The high-temperature deformation behavior of nickel-base superalloy IN718 was investigated in the solution-treated (ST) condition. High-temperature tensile tests were performed between 600 °C and 850 °C at strain rates of 1 × 10−3, 1 × 10−2, and 1 × 10−1 s−1. The deformation behavior of this material was analyzed using optical microscopy, scanning electron microscopy, and transmission electron microscopy. In the investigated temperature–strain rate regime, material undergoes partial precipitation, serrated yielding, and embrittlement. Serrated yielding was observed at 600 °C, 650 °C, and 700 °C and is attributed to dynamic strain aging. The appearance of serrated flow at high temperatures up to 700 °C in the ST condition can be attributed to the availability of excess Nb in the matrix. Beyond 700 °C, Nb concentration significantly decreases in the matrix due to the formation of Ni3Nb precipitate. Nb is responsible for the appearance and disappearance of serrations at high temperature. The alloy exhibits embrittlement phenomenon in the range of 750 °C to 850 °C when thermally exposed in air. The alloy shows a ductile mode of fracture when tested at 600 °C and 700 °C, whereas completely brittle fracture was observed at the 800 °C test temperature. Formation of brittle oxides at grain boundaries in the presence of atmospheric oxygen resulted in the embrittlement of the alloy at 750 °C to 850 °C. An oxidation-assisted intergranular cracking mechanism is responsible for embrittlement of this alloy, which was proved by scanning transmission electron microscopy-energy dispersive spectroscopy.

In the present study, plane strain compression was used to investigate the large plastic deformation behavior and microstructural evolution in magnesium alloy AZ31 with a view to understand the microstructural restoration mechanisms under the applied test conditions. While the stress–strain curves at all test conditions were indicative of dynamic recrystallization (DRX), the amounts of flow softening under different temperature–strain rate conditions was found to be different. The extent of recrystallization increased with increasing deformation temperature or decreasing strain rate with microstructures having higher extent of recrystallization revealing more homogeneous grain structure. DRX was found to be discontinuous in nature and a slightly different mechanism of DRX is proposed based on electron backscattered diffraction (EBSD) analysis. This consists of an inward bulging of the boundary due to the dislocation density being higher inside the larger deformed grains. Consequently, the deformation-induced dislocations outside such inward bulges help in anchoring them leading to the formation of recrystallized nuclei.

The Cu-Cr-Zr-Ti alloy was subjected to single-hit plane strain compression tests in the temperature range of 923 K to 1073 K (650 °C to 800 °C) and at two different strain rates of 0.1 and 1 s−1. These tests were performed to ascertain the kinetics of microstructural evolution resulting by dynamic recrystallization (DRX) in the alloy under hot deformation conditions. The differences in the extent of DRX under various deformation conditions were established by plotting fractional softening as a function of strain and time. Further, the Avrami exponent was found to decrease with increase in deformation temperature. This was attributed to early onset of recrystallization leading to subsequent work hardening of the recrystallized grains and grain growth at higher temperatures which results in the loss of nucleation sites due to reduced grain boundary area. Microstructural observations made using optical microscopy, scanning electron microscopy (SEM), and electron backscattered diffraction (EBSD) revealed nucleation to occur in the following steps: elongation of grains perpendicular to the compression direction, bulging of elongated grain boundaries due to strain-induced boundary migration (SIBM), and subsequent subgrain formation behind the bulged boundary due to strain-induced low-angle boundaries. Finally, the equation describing the DRX kinetics under plane strain conditions for the alloy was established as $$ X = 1 - \exp \left[ { - 0.455 \times \left( {{{\left( {\varepsilon - \varepsilon_{\text{c}} } \right)} \mathord{\left/ {\vphantom {{\left( {\varepsilon - \varepsilon_{\text{c}} } \right)} {\varepsilon_{\text{p}} }}} \right. \kern-0pt} {\varepsilon_{\text{p}} }}} \right)^{2.84} } \right] $$ .

The present work reports the synthesis of Al-X wt.% Si (X = 20, 30, 40 and 50) alloys by a unique combination of pressureless sintering (PLS) and hot forging at 800 and 1000 MPa forging pressures with 37.5 and 50% deformation, respectively, in a specially designed die. The effect of hot forging parameters on densification, hardness, compressive strength and microstructures of the alloys was studied as well as suitably compared with the conventional PLS alloys. The ‘PLS + forged’ alloys, specifically 50% deformed at 1000 MPa, yielded excellent densification and subsequent very good mechanical properties. The combination of hot forging pressure and temperature enabled the high densification due to pore collapse, fracturing of Si particles leading to Al flow between the fractured Si particles and better interfacial diffusion between Si and Al. Moreover, the alloys showed excellent electrical conductivity (~67% of that of pure Al), low coefficient of thermal expansion (CTE) and high thermal conductivity (TC) at par with the same alloys prepared with other methods, like spark plasma sintering, hot pressing and Osprey techniques.

Two different processing routes of mechanical alloying followed by the spark plasma sintering (powder metallurgy) and vacuum arc melting (casting route) were employed to understand the role of processing routes on the phase and microstructural evolution in an equiatomic CrMoNbTiW refractory high-entropy alloy. Besides a major BCC solid solution, a small fraction of carbide, σ phase, nitride, and oxide phases were observed in the alloys prepared by the powder metallurgy route in contrast to a single-phase BCC solid solution in the casting route. The milling atmosphere (dry milling in air and Ar) has significantly influenced the phase and microstructural evolution, illustrating the substantial role of contaminants. Good thermal stability of microstructure at high homologous temperatures was shown based on the long-term heat treatment at 1300 °C for 240 h. The phase evolution predictions via Calphad studies were found to be in reasonable agreement with the experimental observations, albeit with some limitations.

The serrated flow behavior of Al-Li, Al-Cu and Al-Cu-Li alloys was studied in solution-treated condition under a range of strain rates at ambient temperature. The serration behavior and the decrease in stress (i.e., the stress drop) in serrated flow curves are found to be dependent on both the magnitude of the strain rate and the nature and amounts of alloying elements present in these alloys. The underlying mechanisms behind the occurrence of serrations in these alloys are understood to be due to the interaction of solutes, both Cu and Li, with the dislocations.

55Ni-17Cr-12Fe-9Mo-2Nb-1.5Al alloy is a nickel-based superalloy (Russian designation is XH55MбЮ or KhN55MBYu, XH55) without any equivalent in American/European alloy designation. It is used in cryogenic engine of satellite launch vehicles application in two different heat-treated conditions: (1) standard aged (STA) at 730 °C/15 h + 650 °C/10 h and (2) STA + BC (brazing cycle) treatment carried out in vacuum at 1030 °C with holding time of 30 min. Due to the braze cycle adopted for manufacturing, it is essential to study the deterioration in mechanical properties, if any. Hence, the present work is carried out to understand the material behavior in tensile mode (25, 425, 575, 700 and 900 °C) for XH55 alloy in STA condition and STA + BC conditions, compared with corresponding microstructural analysis, morphology and composition using microscopy at various length scales. The tensile stress–strain curve shows characteristic sudden drops in stress with respect to strain, attributed to dynamic strain aging at different temperatures for both STA and STA + BC conditions. In STA condition, the yield strength of the material decreased with increase in temperature. In STA + BC condition, the yield strength decreased up to 425 °C, increased up to 700 °C as the material was subjected to artificial aging during testing and finally decreased at 900 °C. Marginal deterioration in mechanical properties have been observed due to the braze cycle adopted against STA condition.

Mechanical properties at ambient and cryogenic temperatures of Al-Cu-Li alloy are required for design and fabrication of liquid hydrogen and liquid oxygen tanks of satellite launch vehicles. In the present work, bead-on-sheet, friction stir welding was carried out with three different rotation speeds. The yield and strain hardening behaviors of the welds were evaluated in temperature range of 20 K to 298 K. Both yield stress and strain hardening ability in the specimen increased with decrease in testing temperature. The dependence of yield stress on temperature was modeled on the basis of thermally activated dislocation mobility, while that of strain hardening was modeled on the temperature dependence of dynamic recovery rate parameter. The recovery parameter followed an Arrhenius-type relationship with temperature. The model parameters determined from the experimental data were further used to simulate the stress–strain curves at different sub-zero temperatures for the friction stir welds.

The serrations in tensile stress–strain curves for Al-Cu-Li-based AA2198 alloy sheets were studied in various temper conditions. The alloy exhibited typical Portevin-Le-Chatelier effects with intense serrations in the stress–strain curves. This was observed in the solution-treated condition, while the intensity and frequency of serrations decreased upon aging. Detailed microscopic examination together with thermal stability studies showed the absence of δ′ (i.e., metastable Al3Li) precipitate in the alloy. As a result, the shearing of δ’ precipitates by dislocations during plastic deformation did not occur in the present alloy. The quasistatic tensile testing as a function of test temperature, strain rate and temper condition and the acoustic emission study confirmed that the combined interaction of copper and lithium atoms with mobile dislocations is the underlying mechanism for the plastic instability in this alloy. The alloy displayed significant anisotropic behavior in terms of mechanical properties and serration characteristics, and these observations are related to the difference in the effective grain size of the samples in various orientations.

Flux-bounded tungsten inert gas welding is a variant of activated tungsten inert gas welding wherein activating flux is applied on the weld surface with a narrow flux gap along the line of weld. In this study, bead-on-plate welds were performed with flux gaps of 2, 3, 4, 5 and 6 mm using the fluxes silicon dioxide, titanium dioxide and calcium fluoride. The weld bead profiles were obtained using a stereomicroscope from which the depth-to-width ratios (DWRs) were calculated and compared with the DWR of a tungsten-inert-gas-welded plate. The reasons for differences in DWR were explained using the mechanisms involved and the captured images of the welding arc profile. The microstructure of the weld beads revealed no entrapment of flux particles. The increase in the DWR in the presence of activating flux was also substantiated using a numerical simulation model.

The tensile and compressive flow properties, microstructure evolution and fracture behavior at room temperature were investigated in a Ti 6Al 4V alloy, caliber rolled at 750 °C. While pseudo-elastic behavior is exhibited in compression test, the same is not exhibited in the tension test. Pseudo-elasticity is ascribed to the formation of twins and martensite, which also cause a deep and elongated dimple fracture, unlike the conventional dimple fracture in tension. Electron back scatter diffraction studies reveal texture evolution in the compression test but not in the tension test. The tension-compression flow asymmetry and fracture behavior are thus attributed to the difference in microstructure and texture evolution in the two modes of deformation.

Oxide dispersion strengthening is an effective method to improve the high-temperature mechanical properties of the alloys in which various reinforcements are added in the alloy matrix by mechanical milling and consolidating the mixture by different techniques like hot pressing, hot isostatic pressing and spark plasma sintering. This work presents the development of IN 718 ODS alloy by dispersing yttria in IN 718, which has improved mechanical properties at high temperatures. Yttria powder was added to IN 718 alloy powder in different weight fractions of 0.25, 0.5 and 1% and mechanically milled. The composite powders were consolidated by hot pressing and hot isostatic pressing (HIP). A small fraction of isolated porosities in the matrix was observed in hot-pressed samples which caused degradation of the mechanical properties. Percentage of porosities increased with the increase in yttria content. The hot-pressed samples were HIPed to heal the porosities. 0.5 wt% yttria-dispersed Inconel 718 hot-pressed compact exhibited better properties at 850 °C and showed 50 MPa improvement in ultimate compressive strength compared to Inconel 718 alloy.

Commercially pure (CP-70) titanium is used as the metallic liner for composite overwrapped pressure vessels used in satellite launch vehicles. The metallic liner is fabricated from domes machined out of CP-70 titanium forgings. During the hot closed-die forging of CP-70 domes from billets, the amount of deformation varies location to location, due to the non-uniform flow of material in the die cavity. This non-uniform strain distribution in the hemispherical dome results in microstructural variations along the forging axis which will remain even after annealing. The aim of the present work is to study the tensile properties, impact strength and fracture toughness of CP-70 hemispherical forging in annealed condition from top end to bottom end of domes in radial and circumferential directions with a view to understand the variation in mechanical properties to asses the design margin. Study revealed significant amount of the work-hardening difference within a forged dome even after annealing.

Single-wall carbon nanotube (SWCNT) has very high measured elastic modulus. But nanocomposites manufactured using SWCNT does not exhibit dramatic increase in the elastic modulus due to various reasons. One of the defects noticed during the manufacturing of nanocomposites is the agglomeration of SWCNT. A novel methodology is proposed in the present paper to model the SWCNT agglomerates. Subsequently, finite element model of representative volume element (RVE) of nanocomposite is generated using validated boundary conditions and the axial elastic modulus is estimated, which is found to be in close agreement with the experimental results. The validated finite element model is used to estimate other elastic properties of nanocomposites, which can avoid costly experiments. Parametric study is carried out to understand the effect of length, diameter of SWCNT agglomerates and chirality of SWCNT on the elastic properties of nanocomposite. Finally, empirical relations are given to estimate all the elastic moduli of RVE if the diameter (hence chirality) of SWCNT and diameter of SWCNT agglomerates are measured after manufacturing nanocomposite with PVA matrix. The methodology described here can be easily extended for the nanocomposite with any other matrix.

In the present study, the phase evolution and microstructure of CrMoNbTiW, a new equi-atomic refractory high-entropy alloy, are studied. The alloy was synthesized through mechanical alloying (MA) followed by consolidation using spark plasma sintering. After MA, a major BCC solid solution along with residual Cr and Nb were observed. However, secondary phases such as Laves and carbides were also observed in addition to the major BCC solid solution after sintering. Unsolicited contamination from the milling media is found to be one of the reasons for the formation of secondary phases. The high hardness of 8.9 GPa after sintering was attributed to the presence of secondary phases along with the nanocrystalline nature of the alloy. To understand the phase evolution, calculation of phase diagram was carried out using CALPHAD. Further, binary phase diagram inspection and simple empirical parameters were also used to assess their effectiveness in predicting phases.

Compressive ductility of the as-cast AA7075 was nearly doubled by hot compression and by appropriate homogenization treatment. The latter dissolved the 2nd phase, and introduced compositional gradients around the ‘surviving’ precipitates. Direct observations, through split channel die compression, revealed a correlation between particle deformed zone (PDZ: deformed aluminum matrix surrounding coarse 2nd phase particle), the composition gradient and the working temperature. Local misorientation in the PDZ reduced, remarkably, with the introduction of composition gradient and higher working temperature. The generation of PDZ, especially the influence of the composition gradient and the working temperature, was modeled successfully with discrete dislocation dynamics (DDD). More importantly, tailoring the composition gradients around the second phase brought out clear technological possibility of improving the as-cast compressive ductility without sacrificing the particle content and the compressive strength.

Thermal Barrier Coating (TBC) plays a major role in the improvement of gas turbine and engine components in terms of their service life and performance. Generally, all coatings must possess certain primary properties to perform in the intended applications. However, regardless of applications, suitable adhesion strength is one major characteristic they must have to adequately protect the basic components on which they are applied upon. In TBCs, adhesion (or Bond) strength is a parameter that helps to illustrate the resistance of the ceramic top coat against spallation either from the bond coat (and component) or within the TBC layers itself. The performance of TBCs are reliant upon the adhesion between the coating and the metal substrate and also adhesion (or cohesion) between the bond coat and the overlying ceramic top coat layer. The de-bonding of the top coat layer or the inter-metallic bond coat layers are the main reasons of the failure of the overall TBC system. Some of the prominent problems associated with coatings applications are residual stresses, micro-cracks and pores etc. These and many other factors influence the adhesion of the coatings in addition to service environment conditions and pre coating substrate preparations such as substrate cleaning, grit blasting and very importantly plasma spray parameters. In the present work, results obtained from adhesion strength measurements carried out by following the ASTM C 633 standard test method, on various types of TBCs are being shared. Thermal barrier coatings (TBCs) were synthesized with NiCrAlY bond coat deposited on SS 304L substrate by using air plasma spray and different ceramic top coats (a) commercial 8%Yttria Stabilized Zirconia (8YSZ) (b) lab synthesized plasma spray powders of (i) Lanthanum Zirconate (La2Zr2O7) (ii) Lanthanum Ceria Zirconate (La2 (Zr0.7Ce0.3)2O7) and (iii) Lanthanum cerate (La2Ce2O7). The coating depositions were carried out in different configurations i.e. two layers, three layers and gradient layers (Functionally gradient materials). The evaluation of properties includes the studies of morphology of the strength (adhesive/cohesive failure mode) tested specimen as well. General conclusions drawn from the studies on several specimen in various configurations are that cohesive failures (between the ceramic top coat layers) is the predominant mechanisms followed by few adhesive failures in bond coat coat/ceramic interface.

Yttria-stabilized zirconia (YSZ) thermal barrier coatings is most widely used in gas turbine engines applications and its primary role is to protect the underlying base metal from degradation at its high temperature (>1000 °C) service environment. While YSZ serves well in this role, materials with higher thermal stability and lower thermal conductivities are required to be developed for attaining higher operating temperatures and thereby higher energy conversion efficiencies. A number of rare-earth zirconates which form the cubic fluorite-derived pyrochlore structures (A2B2O7) where A: La, Gd, Sm, Ce and B: Zr are being developed, some compositions are more attractive due to their good amalgamation of thermal and mechanical properties. However, when these materials are plasma spray coated on metal substrates, the favorable properties are not immediately realized due to various contributing factors such as poor adhesion/cohesion, microstructure (porosity, defects) or even incomplete stabilization or destabilization of the desired phase (crystal structure) after passing through the plasma. In this paper, plasma sprayable powders of zirconate pyrochlores (or with disordered fluorite structures) synthesized from using La and Ce as the trivalent “A” cation, were plasma sprayed onto Inconel 718 substrates, by using different plasma spray parameters. The considerable influence of these spray parameters on the structural phases (analyzed via XRD) and microstructures (studied via SEM on polished cross section metallographs) are presented in detail.

A combination of low coefficient of thermal expansion (CTE) and decent thermal conductivity (TC) is the reason for the Al-high vol% Si system to become popular for electronic packaging material. In the present work, two process routes, firstly conventional powder metallurgy and then spark plasma sintering (SPS) were utilized for the fabrication of Al-20-60 wt.% Si composites. In addition, effect of small fraction of CNT addition on the CTE of Al-20 wt% Si was studied. Effect of process parameters on the consolidation of the composites in terms of densification, microstructure evolution along with fractographic analysis and strength was studied. CTE and TC of the sintered composites were measured and correlated with the densification, percentage of Si and morphologies of the sintered products. Overall, better densification could be achieved in SPS and the Al-30%Si and Al-40%Si composites SPSed at 550 °C showed average CTE values of 14.52 × 10−6/K and 13.36 × 10−6/K, respectively, in the temperature range of 30–200 °C, which were better than some of the existing alloys with higher Si content. Simultaneously, TC values were 114.4 W/mK and 107.12 W/mK, respectively, for the above two SPSed composites.

The effect of temperature and strain rate on the tensile flow behavior of Fe-0.3 pct C-CrMoV grade steel was studied over a wide range of strain rates (10−4 to 10−1 s−1) and temperatures (700 °C to 950 °C). The flow curves of the steel showed typical dynamic recovery (DRV)-type characteristics at low temperature, high strain rate, and dynamic recrystallization (DRX) type at high temperature > 775 °C. Stress regimes with stress exponent (n) of 3.6 to 5.5 for low–high stresses were observed. The ‘n’ values at temperatures of 850 °C and 900 °C were found to be > 4, which correspond to dislocation climb as the rate controlling mechanism. At 950 °C, ‘n’ value was found to be

We have tried to understand the yield behavior of as-cast CrMoNbTiW refractory high entropy alloy using a modified Varvenne solid solution strengthening model. Yield strength (YS) showed a strong temperature dependence in the 1100 – 1300 °C temperature range. The predicted YS values are significantly affected by the temperature-dependent material constants. The absolute value of YS depends on shear modulus, whereas the variation of YS with temperature is affected by the Poisson’s ratio. Our study demonstrates that it is feasible to predict the high-temperature YS using elastic constants from literature besides DFT studies.

Precipitation hardenable stainless steels have become a material of choice for many advanced engineering applications that require high strength and excellent corrosion resistance. Solution treatment temperature is an important parameter that influences the prior austenite grainsize and retained austenite content in this type of steels. Solution treatment has been carried out in a 12Cr–10Ni steel at treatment temperatures ranging from 750 to 1000 °C at 50 °C intervals. Aging was carried out on two sets of solution-treated samples at 250 and 500 °C. Charpy impact tests at room temperature and 77 K were carried out on solution-treated as well as aged samples. Measurement of retained austenite, prior austenite grainsize, and EBSD analysis were carried out to explain the observed phenomena. Solution treatment temperature is observed to have only feeble effect on impact toughness and hardness; however, it considerably affects the content of retained austenite. The aging temperature and thereby the precipitate size were observed to be the major factors that influence cryogenic impact toughness for 12Cr–10Ni steel.

Hot isothermal plane strain compression (PSC) tests were carried out on a 12Cr-10Ni martensitic precipitation-hardenable (PH) stainless steel, in the temperature range of 750-1050 A degrees C, to study microstructural evolution during large strain deformation. The nature of stress-strain curves varies with Zener-Holloman parameter (Z) with specimens deformed at high Z showing steady-state behavior and those deformed at lower Z showing flow softening. Prior austenite grain size (PAGS), d, exhibited a strong correlation to Z showing a bilinear behavior represented as: d = (1803.9)Z (-0.094) for high Z deformation and d = (1456.2)Z (-0.058) for low Z deformation. Based on the above study, it is recommended to thermomechanically process 12Cr-10Ni steel at Z ae 10(22) for obtaining products with good strength-toughness balance.

This paper presents isothermal uniaxial compression test results of M300 grade maraging steel over a wide range of temperatures (900–1200 °C) and strain rates (0.001–100s −1 ) to examine hot deformability and concurrent microstructural evolution. Processing map is generated and indicated the optimum processing parameters in the temperature range of 1125°-1200 °C and strain rate range of 0.001–0.1 s −1 . High values of the efficiency of power dissipation, microstructural observations and EBSD results indicate softening mechanism to be the occurrence of dynamic recrystallisation. Material constants in a constitutive relation are evaluated from the flow stress data useful in computer modelling.